There are various kinds of semiconductor light receiving device. Among them, APD (Avalanche Photo Diodes) are devices that exhibit particularly high light-receiving sensitivity, including a multiplier function.
APD can be roughly classified into a group of devices made of Si and a group of devices formed with compound semiconductors. The devices containing compound semiconductors can be further classified into a group of devices containing GaAs as a substrate and a group of devices containing InP as a substrate. In terms of structures, those devices can be classified into a planar type or a mesa type.
Based on the differences in layer structure and carrier multiplying structure, those devices can be also classified into an electron multiplying type or a hole multiplying type.
Referring now to FIG. 1, a planar-structure semiconductor light receiving device of the electron multiplier type that includes a compound semiconductor is described.
As shown in FIG. 1, the planar-structure semiconductor light receiving device of the electron multiplier type includes a stack structure formed on an InP semiconductor substrate (1) made of a compound semiconductor. This stack structure includes a p-InP buffer layer (2), a p-type light absorption layer (3), an i-type light absorption layer (4), an electric field relaxation layer (5), a multiplier layer (6), an etching stop layer (7), an n-type buffer layer (8), and an n-type contact layer (9) stacked in this order. The n-type contact layer (9) and the n-type buffer layer (8) are removed around a light receiving part until the removal reaches the etching stop layer (7). Ti ions are then implanted to form a guard ring (11), and Zn diffusion is caused in the neighboring area of the guard ring (11) to form a Zn diffusion layer (10). A p-type electrode (13) is formed on the Zn diffusion layer (10). An n-type electrode (12) is formed on the n-type contact layer (9).
In the semiconductor light receiving device shown in FIG. 1, a bias voltage is adjusted by inputting signal light, so as to multiply the obtained photocurrent with the optimum multiplication factor M (M being approximately 10). As a result, a light receiver having a better reception than a p-i-n type semiconductor light receiving device can be formed by virtue of its amplifier function.
An APD receiver that shows a 10 Gb/s high-speed response can be produced by combining the semiconductor light receiving device shown in FIG. 1 with an optimum receiver circuit.
There is a document that discloses a device that includes an InP semiconductor substrate formed with a compound semiconductor, comprises a planar structure, and is of the electron multiplying type, like the semiconductor light receiving device shown in FIG. 1 (see Non-Patent Document 1, for example).
The device disclosed in the above mentioned Non-Patent Document 1 includes a stack structure formed on a semi-insulating InP substrate. This stack structure includes a p-InP buffer layer, a p-InGaAs light absorption layer, a p-InP electric field relaxation layer, a superlattice multiplier layer, an n-InP etching stop layer, an n-InAlAs cap layer, and an n-InGaAs contact layer stacked in this order. The n-InAlAs cap layer is removed in a ring-like shape surrounding the light receiving part. Ti ions are then implanted, and a Zn diffusion region is formed in the vicinity of the Ti-implanted region. A p-type electrode is formed on the Zn diffusion region.
As a patent document filed prior to the present invention, there is a document that discloses a superlattice avalanche photodiode that includes a stack structure formed on a semi-insulating semiconductor substrate or a high-concentration semiconductor substrate of a first conductivity type via a high-concentration semiconductor buffer layer of the first conductivity type. This stack structure includes a low-concentration semiconductor light absorption layer of the first conductivity type, a semiconductor electric field relaxation layer of the first conductivity type, a superlattice multiplier layer, a high-concentration semiconductor cap layer of a second conductivity type, and a high-concentration semiconductor contact layer of the second conductivity type that are stacked in this order. In the region in the vicinity of the light receiving region of this superlattice avalanche photodiode, a first conducting region is selectively formed from the surface to a location at least deeper than the semiconductor electric field relaxation layer of the first conductivity type. Further, an isolation groove structure of the depth equivalent to the thickness of the high-concentration semiconductor contact layer of the second conductivity type and the high-concentration semiconductor cap layer of the second conductivity type is formed in the boundary region between the first conducting region and the high-concentration semiconductor contact and cap layers of the second conductivity type, so that the first conducting region is not brought into contact with the high-concentration semiconductor contact and cap layers of the second conductivity type. In this manner, the surface leakage dark current that is observed in mesa-type pn-junction photodiodes can be reduced. Thus, a superlattice avalanche photodiode with a low dark current and high reliability can be provided (see Patent Document 1, for example).    Patent Document 1: Japanese Patent Publication No. 2762939    Non-Patent Document 1: “A New Planar-Structure InAlGaAs—InAlAs Superlattice Avalanche Photodiode with a Ti-Implanted Guard-Ring”, I. Watanabe, et at al, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 8, NO. 6, Jun. 1996
A high-speed, high-sensitivity semiconductor light receiving device can be constructed by forming a quasi-planar-type and electron-multiplying-type APD that includes the InP semiconductor substrate (1) made of a compound semiconductor as in the above described semiconductor light receiving device shown in FIG. 1.
However, it has become apparent that the device responsiveness might deteriorate when there is a very intensive light input to the semiconductor light receiving device show in FIG. 1.
In a case where the APD is biased, and light with high level is input to the APD while there is only weak light, the multiplication factor M normally becomes smaller, and is maintained stably. In some devices, however, the initial characteristics are degraded with certain possibility. More specifically, there might be an increase in dark current, and a short-circuit failure might be caused. This is because, when a current flows in the in-plane direction in the p-InP buffer layer (2) of the device, the current concentrates on one spot, and local heat generation occurs.
The above mentioned Non-Patent Document 1 discloses the same technique as that utilized in a planar-structure device of the electron multiplying type that includes an InP semiconductor substrate made of a compound semiconductor like the semiconductor light receiving device shown in FIG. 1. However, unlike the technique according to the present invention, the above described technique is not designed to solve the deterioration of the responsiveness of the device when there is a very intensive light input. The above mentioned Patent Document 1 discloses the high-concentration semiconductor buffer layer of the first conductivity type of approximately 0.5 to 1.0 μm in layer thickness formed on the semi-insulating semiconductor substrate or the high-concentration semiconductor substrate of the first conductivity type. However, Patent Document 1 does not suggest any solution for the problem of the deterioration of the device responsiveness caused when a very intensive light is input.
The present invention has been made in view of those circumstances, and the exemplary object of the present invention is to provide a semiconductor light receiving device that includes a quasi-planar-type and electron-multiplying-type structure formed on an InP semiconductor substrate. This semiconductor light receiving device prevents local heat generation, has high-speed, high-sensitivity characteristics even at the time of an intensive light input, and exhibits high resistance to light inputs.